Codegen: add fiber v3 support

[kerak19]

This PR adds Fiber v3 support to oapi-codegen.

While Fiber v3 is not yet production ready per link, any code changes concerning oapi code generation should've been easy to manage.

Added petstore-expanded and minimal-server examples. Updated Readme to contain Fiber v3 paragraph.

[socket-security[bot]]

New and updated dependencies detected. Learn more about Socket for GitHub ↗︎

Package	New capabilities	Transitives	Size	Publisher
golang/github.com/getkin/[email protected]	environment, eval, filesystem, network, unsafe	0	3.46 MB
golang/github.com/gofiber/fiber/[email protected]	environment, filesystem, network, shell, unsafe	0	2.33 MB
golang/github.com/google/[email protected] ➜ v1.6.0	None	0	78.2 kB
golang/github.com/kerak19/fiber-middleware/[email protected]	network	0	47.4 kB

View full report↗︎

[jamietanna]

I'll try and get a review at some point - if Fiber are still pushing out breaking changes to v3 I'd be concerned that this may mean we'll have more work keeping this working 🤔